Shindaiwa has had 4-stroke handhelds in production since 2001. Stihl's 4-mix came out about a year later. Not sure about Honda's micro technology, but it's been out for quite a while.
Three major handheld equipment manufacturers have tested 4-cycle chainsaw prototypes: Dolmar, Shindaiwa and Stihl. Supposedly Stihl was "actively testing" a large displacement, OHC horizontal cylinder 4-cycle chainsaw back in 2007-2008. However, at least in Dolmar's case the technology nor the market was/is ready for it. The main reason is price - in Dolmar's case, you could buy a more powerful, 65cc range 2-cycle chainsaw for the same price. I would assume that this is the same thing that the other manufacturers ran into.
However, since then most if not all of the focus has switched to stratified designs...obviously, Husqvarna and Stihl are both using it already. Dolmar is implementing it, we should be seeing stratified Dolmars before too long. There's a good reason that they are pursuing that instead of 4-stroke. I'd say that's the future of chainsaws, at least for now.